The parasympathetic nervous system (PNS) is one of the two main divisions of the autonomic nervous system (ANS), functioning alongside the sympathetic nervous system (SNS) to maintain homeostasis. The PNS is often described as the “rest and digest” system, as it dominates during periods of relaxation and supports restorative processes.

This guide covers the anatomy, actions, and clinical relevance of the PNS, with detailed breakdowns and explanations for USMLE Step 1 preparation.

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Anatomical Structure

• Pre-ganglionic fibers: Originate from the brainstem and sacral spinal cord, synapsing with postganglionic fibers.
• Peripheral ganglia: Groups of neuronal cell bodies where synapses occur.
• Postganglionic fibers: Carry signals from ganglia to effector organs.

Component	Origin	Myelination	Length	Neurotransmitter Released
Pre-ganglionic fibers	Brainstem, S2–S4 spinal cord	Yes	Long	Acetylcholine
Post-ganglionic fibers	Peripheral ganglia	No	Short	Acetylcholine

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Pre-Ganglionic Neurons

• Origin: Brainstem and sacral segments (S2–S4) of the spinal cord.
• Pathways:
  • Cranial nerves III, VII, IX, and X.
  • S2–S4 spinal nerves (forming the pelvic splanchnic nerves).
• Special Note: The PNS is also called the craniosacral division of the ANS due to its origins.

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Peripheral Ganglia

• Location: Close to or within the target organs.
• Function: Synaptic site where pre-ganglionic neurons communicate with post-ganglionic neurons.

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Post-Ganglionic Neurons

• Origin: Peripheral ganglia.
• Pathway: Transmit signals to effector organs.
• Characteristics: Short, unmyelinated, and release acetylcholine.

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Parasympathetic Innervation

Head and Neck

• Cranial nerves involved: III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus).
• Ganglia:
  • Otic.
  • Ciliary.
  • Pterygopalatine.
  • Submandibular.

Trunk and Limbs

Vagus Nerve (CN X)

• Contribution: ~75% of parasympathetic outflow.
• Target organs: Thoracic viscera and most of the gastrointestinal tract.

Sacral Spinal Nerves

• Origin: S2–S4 anterior rami.
• Formation: Pelvic splanchnic nerves.
• Target organs: Distal colon, rectum, bladder, and reproductive organs.

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Actions of the Parasympathetic Nervous System

System	Action
Eyes	Pupil constriction.
Lacrimal/Salivary glands	Secretion stimulation.
Cardiovascular	Decreased heart rate.
Respiratory	Reduced airway diameter.
Digestive	Increased activity, sphincter dilation, glycogen synthesis, and digestive gland secretion.
Urinary	Bladder contraction and internal sphincter relaxation.
Reproductive	Penile/clitoral erection, vaginal lubrication, and uterine relaxation.

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Clinical Relevance

Truncal Vagotomy

• Procedure: Severing the vagal supply to the stomach to reduce acid secretion.
• Indication: Previously used for peptic ulcer disease.
• Complications: Gastroparesis due to impaired smooth muscle contraction.

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Points to Remember

1. The PNS is the craniosacral division of the ANS.
2. It uses acetylcholine as its primary neurotransmitter for both pre-and postganglionic neurons.
3. The vagus nerve accounts for the majority of parasympathetic outflow.
4. PNS actions include conserving energy, promoting digestion, and facilitating reproductive functions.
5. Clinical conditions like peptic ulcers previously involved interventions targeting the PNS (e.g., truncal vagotomy).